1. Field of the Invention
The present invention generally relates to nozzles for the extrusion of materials and/or application of materials to surfaces and, more particularly, to nozzles for applying a conductive paste to surfaces of substrates to form interconnection boards in a thick film deposition process.
2. Description of the Prior Art
In the construction of electrical devices, the structures for assembling circuit elements into functional groups and the packaging and structural support of such functional groups of circuit elements often constitutes a major portion of the cost of a manufactured electronic device or component therefore. With increased complexity of individual circuit elements and manufactured devices, in general, has come an increased complexity of interconnection structures such as printed circuit boards using, for instance, epoxy and fiberglass substrates. Increased circuit element density as well as integration density has increased heat tolerance and dissipation requirements of these interconnection elements as well as the use of multi-layer boards.
A high-performance structure now in widespread use is the multi-layer ceramic (MLC) type of interconnection structure, described in U.S. Pat. No. 4,245,273, to Feinberg et al., for PACKAGE FOR MOUNTING AND INTERCONNECTING A PLURALITY OF LARGE SCALE INTEGRATED SEMICONDUCTOR DEVICES, assigned to the assignee to the present invention, and hereby incorporated by reference herein. In these structures, a potentially differing interconnection pattern is formed on each of a multiplicity of layers of ceramic substrate. These interconnection patterns include perforations, known as vias, in the ceramic carriers which are selectively filled with conductive paste and provide electrical continuity between layers of the MLC structure. The respective layers are then stacked and sintered under pressure and high temperature to provide a unitary structure with many interconnection lamina embedded therein to allow formation of electronic interconnection structures of high connection complexity. However, testing of each layer as it is formed requires a number of steps at least corresponding to the number of layers and an equal number of geometrical setups to allow testing by automated machinery. For this reason, it is common to optically inspect each layer as it is formed and to test only a portion of each layer for circuit continuity to some degree of confidence. Full electrical continuity is only tested a single time after the entire multi-layer ceramic (MLC) structure is assembled and sintered. This final test would also be required even if the individual layers were individually fully tested, since the electrical connection patterns can be damaged by either of the assembly and sintering steps of the process. Therefore, to achieve reasonably high manufacturing yields it is particularly necessary to form the individual layer by techniques which are productive of extremely low incidence of defects. The conductive patterns must also be formed with high regularity and consistency to reduce susceptibility to the formation of defects during assembly and sintering.
The formation of the conductive patterns on the individual substrate layers is done by assembling a stencil on the substrate, applying a layer of conductive paste and then removing the stencil. This process in commonly known as screening. Once the pattern is initially formed, other processing steps, such as drying, may also be employed to stabilize the pattern during assembly. Then the layers are assembled and, typically, sintered, as indicated above.
The use of a stencil and the fineness of typical interconnection patterns requires a reasonably high pressure to be developed for the application of the conductive paste. At one time, the layer of conductive paste was typically applied by means of a TEFLON.TM. (polytetrafloroethylene) squeegee. As conductive patterns increased in complexity, however, a squeegee was not able to develop sufficient pressures to reliably penetrate the stencils with the conductive paste to form the desired interconnection patterns and to reliably fill the via perforations in the ceramic carriers. Accordingly, it is presently deemed desirable to apply the conductive paste with a nozzle which confines the conductive paste against the substrate and allow the production of a uniform layer on the substrate and full stencil penetration. A description of a screen printing method for forming MLC structures using a nozzle is described in U.S. Pat. No. 4,808,435, to Cropp et al., assigned to the Assignee of the present invention and which is hereby incorporated by reference.
Since a substantial pressure is applied to the paste in the screen printing process, it is necessary to apply an adequate force to the nozzle to press the nozzle against the stencil in order to confine the paste and maintain the paste pressure. This pressure of the nozzle against the stencil, which is typically formed of a nickel sheet, results in friction as the nozzle is moved relative to the stencil. The friction against the stencil also causes wear of the nozzle and, for that reason, it is desirable that the nozzle have a surface which is resistant to abrasion. A nozzle formed of Tungsten Carbide is disclosed SCREENING NOZZLE WITH NON-LINE CONTACT by R. C. Brilla et al. in IBM Technical Bulletin, Vol. 25, No. 6, November 1982, which is also incorporated by reference, herein. This publication also addresses the issue of nozzle profile in order to maintain confinement of the paste where the stencils have relatively coarse features relative to the area of nozzle contact with the stencil.
This potential problem can also be solved in the manner disclosed in U.S. Pat. No. 3,384,931, to Cochran et al., assigned to the assignee of the present invention and also incorporated by reference, using a more complex stencil structure. In this latter method and apparatus, the stencil is fabricated in a manner where the stencil features are formed on only one side of the stencil and the stencil is perforated by many tiny apertures to allow penetration of the paste to the stencil feature area and escape of air from the volume formed by the stencil feature. This stencil structure and technique cause a point contact of the nozzle with the stencil to be preferred. A point contact allows a narrow pressure angle to be formed by essentially tangent contact between the nozzle and the stencil to increase effective screening pressure. The viscosity of the conductive paste effectively maintains pressure of the paste as the stencil features are filled and no further air is present in the stencil feature.
This latter method of screening and stencil structure is presently preferred. The more complex stencil structure, however, makes it desirable that wear and scoring of the stencil be minimized. To a certain degree, stencil degradation is reduced by the point contact of the nozzle. However, as the nozzle itself becomes worn or otherwise degraded due to both friction with the stencil and interaction with the conductive paste, the nozzle profile becomes more flat and the contact area becomes wide; increasing the tendency for scoring of the stencil to occur.
To maximize stencil life, a rework limit has been established for the nozzles to control the screened line dimension, scoring levels and pattern quality. When a nozzle profile reaches a predetermined dimension of flat surface, the nozzle is reworked to obtain a round profile and then reused. However, during nozzle degradation, a significant amount of scoring of the stencils nevertheless occurs; resulting in scrapping of a stencil typically after much less than two thousand passes.
The short useful life of the stencils as well as the complexity thereof significantly increases the cost of the screening process and less than optimum screened pattern quality. In addition, the substrate or carrier can become contaminated by metal shavings resulting from the scoring. In this regard, it has been found that tungsten carbide resists picking up materials from the mask or stencil which may cause scoring. Scoring also degrades the screening process since it degrades the tightness with which the nozzle fits against the stencil and causes smearing of the conductive paste as well as potential loss of pressure.
Although, as pointed out above, tungsten carbide has been proposed for use in some nozzles having a flat profile, the manufacture of nozzles from this material is expensive. Therefore, it has been deemed preferable to fabricate the nozzles from steel. As an enhancement to the steel nozzle, a tungsten carbide coating has been applied to the steel of the nozzle tip by flame spraying. However, it was found that during thick film deposition, the carbide coating would disintegrate, resulting in the original nozzle structure. The nozzle rework loop in the thick film process was also expensive because of the step of rejuvenating the carbide coating.
Accordingly, there is a need in the thick film deposition process and other coating, extrusion and screening processes, for a nozzle structure which is of reduced expense to manufacture and use and which would resist degradation by having a tip construction which is of a material much harder than the paste constituents and the stencil material but which would be smooth and resist collection of material from the stencil in order to achieve increased mask or stencil pass factors and decreased nozzle rework levels and expense.